1. Field of the Invention
This invention generally relates to integrated circuit (IC) and liquid crystal display (LCD fabrication and, more particularly, to thin-film transistors (TFTs) formed on a metal foil substrate and a process for forming the same.
2. Description of the Related Art
High quality polycrystalline silicon material is the building block of high performance TFTs that are used in integrated circuits and microelectronic devices such as LCD""s. The higher the quality of the poly-Si material, that is, the closer to single-crystal Si material, the better the performance of the resultant devices. Therefore, it is desirable to develop methods that yield high quality polysilicon (poly-Si) material for display or other electronic products.
The performance of the device is affected not only by the crystalline quality of the active layer, but also by the quality of the gate insulator film that covers the active layer. Both the bulk properties of the gate insulator, as well as the properties of the interface that forms between the gate insulator and the poly-Si layer, are very important for the operation of the device. For Si or poly-Si devices, the best gate insulator film is SiO2, and the best method of forming a high quality SiO2 film with excellent bulk and interface properties is by thermal oxidation.
A silicon substrate has a sufficiently high melting point to withstand thermal treatments up to temperatures in the range of 1200xc2x0 C. Thus, thermal oxidation at 900-1150xc2x0 C. is possible on silicon wafers. When the substrate, however, is made of glass or plastic, as is typically the case for LCDs and/or flexible/conformable microsystems, the maximum process temperature window is restricted to much lower temperatures.
The use of alternative substrate materials is of interest, as it would enable the realization of new products that are not otherwise feasible to make. One particular aspect of interest is flexibility, the ability of the microsystem to bend, conform, or maintain its integrity under external xe2x80x9cstressxe2x80x9d. These attributes would enable the manufacturing of a variety of one-use products and/or the manufacturing of robust products that maintain their functionality under a wide range of external, xe2x80x9cenvironmentalxe2x80x9d conditions. Therefore, there is motivation to develop microsystems, such as displays with electronics, sensors, or other products that combine TFT microelectronic devices, that are robust, have high performance, and are cheap to make.
Very high performance transistors can be made on various substrates using laser annealing technology. However, this technique is typically much more expensive than solid-phase-crystallization (SPC). The latter, however, lacks the performance of laser annealing, as the annealing temperatures must be restricted when glass substrates are used.
It would, therefore, be advantageous if a technology were available that could utilize solid-phase crystallization, but offer the performance levels of laser annealing in the fabrication of TFTs.
It would be advantageous if the above-mentioned high-performance TFTs could be fabricated on a flexible substrate for use in flexible microsystems.
The present invention describes a technology that enables the fabrication of high performance devices for flexible microsystem applications, using a standard, low cost poly-Si TFT process flow. One aspect of the invention is the combination of high temperature thermal oxidation with solid-phase-crystallized poly-Si material. Thermal oxidation requires temperatures in the range of 900-1150xc2x0 C., which is not compatible with conventional flexible substrates. This problem is solved in the present invention by utilizing flexible thin metal foils as the starting substrate. Thin metal foils can withstand temperatures in excess of 1000xc2x0 C. if certain treatments are applied the initial metal foil material.
Accordingly, a method for is provided forming a thin-film transistor (TFT) on a flexible substrate. The method comprises: supplying a metal foil substrate such as titanium (Ti), Inconel alloy, stainless steel, or Kovar, having a thickness in the range of 10 to 500 microns; depositing amorphous silicon; annealing the amorphous silicon to form polycrystalline silicon; and, thermally growing a gate insulation film overlying the polycrystalline film.
The amorphous silicon annealing process can be conducted at a temperature greater than 700 degrees C. using a solid-phase crystallization (SPC) annealing process. Thermally growing a gate insulation film includes: forming a first film polycrystalline silicon layer having a thickness in the range of 10 to 100 nanometers (nm); and, thermally oxidizing the first film layer at temperature in the range of 900 to 1150 degrees C. for a period of time in the range of 2 to 60 minutes.
Alternately, thermally growing a gate insulation film further includes plasma depositing a second layer of oxide overlying the first film. Then, the first film has a thickness in the range of 10 to 50 nm and the second layer of oxide overlying the first film has a thickness in the range of 40 to 100 nm.
Additional details of the above-described method, and a thin-film transistor on a flexible substrate are provided below.